When Perl goes to close the $H handle, it will wait for the running command to finish. Because 'tail -f' never ends, it waits forever. I'd suggest using File::Tail here, but you could also kill the tail yourself. The call to open will return a PID that you can kill when MainLoop is finished.

my $tail_pid = open($H,"tail -f -n 50 z.txt |") or die $!;

# etc.

MainLoop;
kill 9, $tail_pid;
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